The present invention relates generally to automatic assignment of physical cell identities in a mobile communication network and, more particularly, to the detection and correction of conflicts between physical cell identities for neighboring cells in a mobile communication network.
In a mobile communication network, the coverage of the network area is divided into a plurality of cells. Each cell is identified by a unique physical cell identifier (PCID), which is broadcast to the user terminals over a broadcast channel. The PCID enables the user terminals to differentiate between different cells when implementing cell search algorithms for mobility management purposes. Conventionally, a network administrator assigns the PCIDs to the cells so that neighboring cells do not have the same PCID.
One of the goals of the Long Term Evolution (LTE) standard currently being developed by the Third Generation Partnership Project (3GPP) is to simplify deployment of LTE networks. One way to simplify deployment is to make the network self-configuring in as many aspects as possible. Two related areas where self-configuration is being considered are automatic assignment of PCIDs and automatic generation of neighbor cell relation (NCR) lists.
When PCIDs are assigned without manual planning, there is some risk of PCID conflicts. A PCID conflict exits when two neighboring cells have the same PCID. Such conflicts could result in mobility management errors, such as dropped calls. Accordingly, a mechanism is needed to resolve such conflicts.
Most PCID conflicts will manifest themselves as ambiguities in NCR lists. Thus, a PCID conflict can be detected if one cell has two neighbors in the NCR list with the same PCID and Public Land Mobile Network Identity (PLMNID) on the same carrier frequency; but a different Global Cell Identity (GID). This method of detection requires a third cell to detect conflicts. Thus, cells with conflicting identities need help from on a third, common neighboring cell to detect the conflict. As a result, one or both cells must change its old conflicting PCID to a new PCID.
There are some circumstances when PCID conflicts will not be revealed in the NCR list and therefore not detected by a third cell. For example, in the case of a small isolated micro cell inside of a larger macro cell, there may not be a third cell that has both the micro cell and the macro cells as neighbors. Also, a third neighboring cell may not detect a PCID conflict when the conflicting cells are isolated (e.g., an island with only two cells). A “highway deployment” where several cells are aligned along a road such that each cell only has two neighbors may also prevent detection of the PCID conflict by a third cell.
PCID transmission gaps have been proposed as one method of detecting PCID conflicts in situations where the conflicts can not be detected as ambiguities in NCR lists. The basic idea is to stop transmitting all or some reference signals corresponding to the PCID at predefined (e.g., pseudo-random) times and to order the connected user terminals to try to detect during the gap if any other cell is transmitting reference symbols corresponding to the same PCID. During normal operation, there is no need to issue a reference signal transmission gap. The PCID is a static property of a cell that is initialized when the cell first goes into operation and then that value is typically never changed during the lifetime of the cell. Reference signal transmission gaps are therefore most useful in newly installed cells that need to verify that the initial PCID is locally non-colliding. Reference signal transmission gaps could however also be used infrequently (e.g., once a day) to verify that the used PCID is still locally non-colliding. Especially in small micro cells with few neighboring cells this might be a good safety measure.